Method for forming mirror-reflecting film in optical wiring board, and optical wiring board

ABSTRACT

An aspect of the present invention is directed to a method for forming a mirror-reflecting film on a waveguide in an optical wiring board, characterized in that a multilayer film, in which a base, a metal layer and an adhesive layer are layered in this order, is used, and the metal layer is transferred and bonded to an inclined face for mirror-reflecting film formation provided on the waveguide, with the adhesive layer of the multilayer film intervening. The present invention provides a method which, when forming a mirror-reflecting film on a waveguide in an optical wiring board, enables inexpensive and easy formation of the mirror-reflecting film, using the smallest quantity of metal possible and employing comparatively simple facilities and techniques.

TECHNICAL FIELD

This invention relates to a method for forming a mirror-reflecting filmin an optical wiring board, and to an optical wiring board comprisingthe mirror-reflecting film formed by this method.

BACKGROUND ART

Optical wiring boards, which are printed boards incorporating opticalwaveguides, are attracting attention as a means of resolving suchpersistent problems in various information processing equipment ashigh-frequency noise accompanying faster signals and inadequatetransmission bandwidth.

In such optical wiring boards, mirror-reflecting films are formed onoptical waveguides in order to bend light at desired angles, as forexample when light is output from or, conversely, input to the boardsurface. As the method of forming such films, for example a method suchas that of Non-patent Document 1 is known. This method comprises aprocess of forming a mirror-shape inclined face, and a process offorming a reflecting film on this surface. A summary of the method isexplained as follows, referring to the figures.

In FIG. 1, (a) through (i) are schematic diagrams for explaining anexample of a method of fabricating an optical wiring board, in which amirror-reflecting film is formed on an optical waveguide, and show arepresentative method of fabricating a flexible-type optical wiringboard.

First, a flexible electric board comprising a metal layer 1 of copperfoil or similar and an insulating layer 2 formed from polyimide resin orsimilar is prepared ((a) of FIG. 1). On the face of the insulating layer2 is formed a first cladding layer 3 ((b) of FIG. 1).

As the material of the cladding layer 3, resin having the desiredtransparency at the wavelength of the propagating light, for example 850nm, is used. Various forms may be used, including liquid,semi-solidified film, or a UV-hardening material or thermosettingmaterial. When a liquid is used, for example a spin coating method isapplied, or in the case of a film, for example a vacuum laminationmethod is applied, and the liquid or the film is deposited on theinsulating layer 2 and is hardened as necessary.

Next, a core layer 4 is formed ((c) and (d) of FIG. 1). The core layer 4is a portion which confines light and enables propagation of lightthrough total reflection at the interface with the cladding layer 3, andis normally patterned to a width of order several μm to several hundredμm.

As the constituent material of the core layer 4, a resin having a higherrefractive index than the cladding layer 3, and having the desiredtransparency at the wavelength of the propagating light, for example 850nm, is used. The form may be liquid or a semi-solidified film, and ingeneral, a material which has UV hardening properties and can bepatterned by UV lithography is employed.

The core layer 4 is generally formed by steps in which, after the entiresurface is covered with film similarly to the cladding layer 3, maskingis performed to mask unnecessary portions, and UV irradiation is thenperformed to harden only the necessary portions ((c) of FIG. 1),followed by a step in which the unnecessary portions are washed away(development) ((d) of FIG. 1).

Next, an inclined face 5 for mirror film formation is formed ((e) ofFIG. 1). As the technique, for example a method employing a dicingblade, a method employing a router blade, or a method employing a lasermay be used, in which normally the inclined face 5 is formed at an angleof approximately 45°. In the step immediately after formation of theinclined face 5, it may occur that the surface smoothness isinsufficient for use as a mirror; in such cases, a varnish obtained bydiluting the waveguide material may be applied to the machined face toimprove the smoothness.

Next, the mirror-reflecting film 6 is formed on the inclined face 5 ((f)of FIG. 1). In forming the mirror-reflecting film 6, normally a vacuumprocess such as vacuum evaporation or sputtering is adopted. As thematerial of the mirror-reflecting film 6, of course a material withsuperior reflectivity in the wavelength region of the light to betransmitted is selected, but in consideration of reliability and cost, amaterial with a balance of properties is selected. For example, inNon-patent Document 1, Au (gold) is used.

Next, the core layer 4 in which the mirror-reflecting film 6 has beenformed on the inclined face 5 is covered with a second cladding layer 7((g) of FIG. 1). In forming the second cladding layer 7, in general, thesame material as in the first cladding layer 3 is adopted. The techniqueis also similar; in the case of a liquid, for example a spin-coatingmethod is used, and in the case of a film form, for example a vacuumlamination method can be used for the formation (film deposition),followed by hardening performed as necessary.

After forming the second cladding layer 7, a coverlay layer 8 is formedthereupon to protect the light transmission layer ((h) of FIG. 1). Asthe material of the coverlay layer 8, a polyimide resin, polyesterresin, or similar is used, and is applied by a vacuum press method orlamination method, followed by heating at approximately 140 to 170° C.to harden an adhesive layer.

Finally, processes such as formation of through-holes, circuitpatterning, and solder resist, as well as metal plating are performed tocomplete the surface layer circuit 9 ((i) of FIG. 1). At this time,micrometer-order positioning precision may be required for the pads formounting optical elements, and in such cases, a method of high-precisionpad formation using laser machining is adopted, as for example describedin Patent Document 1.

-   Patent Document 1: Japanese Patent Application Laid-open No.    2007-086210-   Non-patent Document 1: Matsushita Electric Works Technical Report,    Vol. 54, No. 3, “Optical/Electric Composite Flexible Print Board”    (issued September 2006)

SUMMARY OF THE INVENTION

In the conventional methods of fabrication of optical wiring boards suchas that shown in FIG. 1, as the method of forming mirror-reflectingfilms 6 on optical waveguides, vacuum evaporation methods or sputteringmethods as described above have been adopted to cause a metal to adhereto an inclined face 5. However, according to an investigation by theinventors, vacuum processing equipment or other large-scale equipment isnecessary in order to perform such methods, so that excessive economicburdens are imposed.

Moreover, according to the investigation by the inventors, metal cannotbe propelled only onto limited necessary areas in these methods, so thatadhesion of the metal is to be performed in a state covering regionsother than the regions for adhesion by masking, thus making the adhesiontask difficult. In addition, the metal evaporation source (sputteringsource) is used in greater than the needed amount, so that there is theproblem that material costs are increased.

In view of the above problems residing in the conventional methods, anobject of the present invention is to provide a method enabling easy andinexpensive formation of a mirror-reflecting film, using the smallestquantity of metal possible and employing comparatively simple facilitiesand techniques, when forming mirror-reflecting films on waveguides inoptical wiring boards.

An aspect of the present invention is directed to a method of forming amirror-reflecting film on a waveguide in an optical wiring board, themethod being characterized in that a multilayer film, in which a base, ametal layer and an adhesive layer are layered in this order, is used,and in that the metal layer is transferred and bonded to an inclinedface for mirror-reflecting film formation provided on the waveguide,with the adhesive layer of the multilayer film intervening.

Another aspect of the present invention pertains to an optical wiringboard comprising the mirror-reflecting film fabricated by the methoddescribed above.

The objects, features, aspects and advantages of the present inventionwill become more apparent from the detailed description below along withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic process diagram showing an example of aconventional method of fabricating an optical wiring board in which amirror-reflecting film is formed on an optical waveguide.

FIG. 2 is a cross-sectional diagram showing an example of theconfiguration of the multilayer film for transfer in an embodiment ofthe invention.

FIG. 3 is a schematic process diagram showing an example of the methodof fabricating the multilayer film in an embodiment of the invention.

FIG. 4 is a conceptual diagram showing an example of the method whenhardening the adhesive layer during transfer.

FIG. 5 is a conceptual diagram showing an example of the method whenhardening the adhesive layer after transfer.

FIG. 6 is a conceptual diagram showing an example of the transfer methodusing a head with an elastic tip.

FIG. 7 is a diagram showing an example of the preferred multilayer filmfor transfer.

FIG. 8 is a diagram showing an example of the method of fabricating thepreferred multilayer film in which the metal layer is divided.

FIG. 9 is a diagram showing a manner in which light passes through theadhesive layer during mirror reflection.

DESCRIPTION OF EMBODIMENTS

Below, embodiments of the invention and advantageous effects of theinvention are explained in greater detail, referring to the drawings.

A characteristic of the present invention is that, in a well-knownprocess of fabricating an optical wiring board such as for example thatshown in FIG. 1 above, an innovation has been made in the method offormation of the mirror-reflecting film in particular. Specifically, asexplained in detail below, the invention is characterized in that amultilayer film in which a base, a metal layer and an adhesive layer arelayered in this order is used; the multilayer film is set such that theadhesive layer is in contact with an inclined face for a mirror which isformed on the waveguide in the optical wiring board; and thereafter thebase alone is peeled to cause transfer and bonding of the metal layer tothe inclined face for the mirror.

FIG. 2 shows the configuration of the multilayer film for transfer, inwhich fundamentally a three-layer structure comprising a base A, metallayer B, and adhesive layer C is employed.

No limitations in particular are imposed on the material of the base A,and a polyester resin such as polyethylene terephthalate (PET), anacrylic resin, a polyimide resin, and the like can be used. It ispreferable that the material be easily peelable from the metal layer Bwith which the base is layered, and ordinary plastic film is generallyappropriate.

When the adhesive layer C which is used is a material requiring heatingfor transfer, such as for example a thermosetting resin or a hot-meltresin, the base A can be selected from materials with thermalresistance, having the ability to withstand the heat used in transfer.No limitations in particular are imposed on the thickness of the base A,but it is preferable that the layer be thin in order to secure theflexibility to enable the base to follow the inclined face for mirrorformation, and a thickness of 0.2 mm or less is recommended. Thethickness of the base A is preferably in, but not limited to, a range of0.01 to 0.05 mm.

No limitations in particular are imposed on the type of the metal layerB either, but it is preferable to use a metal having high reflectivityfor the light to be transmitted. For example, 850 nm is a representativewavelength of VCSELs, which are light-emitting elements suitable forsurface mounting. Metals with a high reflectivity at this wavelengthare, for example, copper, silver, gold, aluminum, nickel, chromium, andsimilar. When the stability of the material is considered, copper,silver and gold are particularly preferable, and gold in particular isoptimal. However, of course another metal can be selected, taking intoconsideration the environment of use including temperature and humidity,or required characteristics.

The thickness of the metal layer B need only be such that transmittedlight does not penetrate, and a target thickness of approximately 0.1 μmor greater is sufficient. The thickness of the metal layer B ispreferably in, but not limited to, a range of 0.1 to 0.5 μm.

The adhesive layer C is a layer which is indispensable for transferringthe metal layer B onto the inclined face for mirror formation, and anymaterial which hardens can be used. Hardening types of materials includeUV hardening types and thermosetting types; specifically, epoxy resins,acrylic resins, polyester resins, and the like are preferable examples.In order to obtain excellent close adhesion with the constituentmaterial of the waveguide, it is preferable that a material of the samekind as the waveguide constituent material be used.

When a UV hardening type is used, it is desirable from the standpoint ofprocess machining properties that hardening occurs at an irradiationamount of approximately 4000 mJ or less. In the case of a thermosettingtype, materials which harden at 60° C. to 150° C. are easy to handle,and so are preferred.

The thickness of the adhesive layer C may be approximately 1 μm orgreater. It is noted that when the inclined face for mirror formation isformed using dicing or other normal machining, cutting bands may appearin the machined face, and so the thickness of the adhesive layer C ispreferably thicker than the maximum difference of height between the topand bottom surface of such cutting bands or other roughness. This isbecause cutting bands and similar can be reliably filled in, so that asmooth reflecting face can be obtained. By this means, pretreatment ofsmoothing the face for reflecting film formation is made unnecessary,and optical scattering losses of the mirror-reflecting film can besuppressed. The thickness of the adhesive layer C is preferably in, butnot limited to, a range of 1 to 3 μm.

It is desirable that the refractive index of the adhesive layer C besubstantially the same as that of the core layer. The reason isexplained as follows. When the metal layer B is transferred onto theinclined face for mirror formation, light must penetrate the adhesivelayer C in order for the light to enter and leave the waveguide. Ifthere is mismatching of refractive indexes, light reflection lossesoccur according to the magnitude of the mismatch. However, by making therefractive index substantially the same for the core layer and for theadhesive layer C, such light reflection losses can be prevented.

Similarly, it is desirable that the adhesive layer C be transparent inthe wavelength range of light being transmitted. This is because, asexplained above, since the light passes through the adhesive layer Cwhen reflecting light, the absorption losses occur if the adhesive layerC is not transparent.

No limitations in particular are imposed on the method of manufacture ofthe multilayer film F as well. For example, the method shown in theprocess diagram of FIG. 3 may be used. That is, the base A such as a PETfilm is prepared ((a) of FIG. 3), and vacuum evaporation, sputtering, oranother arbitrary method is used to form the metal layer B having thedesired thickness on the surface of the base A ((b) of FIG. 3). Then, onthe surface of the metal layer B, a varnish containing the material tobecome the adhesive layer C is formed to the desired thickness bypainting, spin-coating, dipping, or another arbitrary method ((c) ofFIG. 3), and if necessary is dried ((d) of FIG. 3).

Next, a method of transferring the metal layer B of the multilayer filmF onto the inclined face for mirror formation is explained. Thisinvention is characterized by a method of forming a metal layer on aninclined face for mirror formation. Processes up to formation of aninclined face for mirror-reflecting film formation on a waveguide in anoptical wiring board, and processes from formation of a second claddinglayer after reflecting film formation up to surface circuit formation,can be performed similarly to (a) to (e) of FIG. 1 and (g) to (i) ofFIG. 1 in the technology of the prior art explained in FIG. 1 above.Hence an explanation of these processes is omitted, and the explanationis focused on the processes for forming the metal layer B.

FIG. 4 is a conceptual diagram showing an example of the method whenhardening the adhesive layer during transfer, in which both a case ofthermosetting and a case of UV hardening are indicated. In the case ofUV hardening, only a method of using a mask to limit the irradiatedregion is shown.

As shown in (a) of FIG. 4, while confirming the position using amicroscope or other means, the multilayer film F comprising the base A,metal layer B, and adhesive layer C is set on the surface of theinclined face 5 for mirror formation, to which transfer is to beperformed. Then, as shown in (b), (b-1), and (b-2) of FIG. 4, a head (orheating head) H, preferably with a tip portion comprising elasticmaterial, is brought close to the side of the base A of the multilayerfilm F, and the multilayer film F is pressed against the inclined face5. The pressing force at this time, although it depends on the materialcharacteristics of the adhesive layer C, is preferably in the range of0.05 M to 0.2 MPa from the standpoint of process attributes.

When hardening the adhesive layer C, it is preferable that the adhesivelayer C is hardened while the multilayer film F is fixed with the headH, from the viewpoint of stability. When the adhesive layer C hasalready adhesive properties, or exhibits adhesive properties uponheating, the adhesive layer C can be hardened in a state that the head His removed.

When the adhesive layer C is a thermosetting type layer, as shown in (b)of FIG. 4, the heating head H is pressed from the side of the base Aagainst the inclined face 5, and heat and pressure are applied. In thiscase, because it is necessary to convey heat from the heating head Hthrough the base A and metal layer B to the adhesive layer C, theheating temperature is preferably set to the temperature of 1 to 20%higher than the thermosetting temperature. This is because aconsiderable amount of heat is dispersed from the metal layer B.

When the adhesive layer C is a UV-hardening layer, upon pressing thehead H against the inclined face 5 from the side of the base A as shownin (b-1) of FIG. 4, the adhesive layer C is irradiated with UV from therear-face side as shown in (b-2) of FIG. 4 and is hardened. This isbecause, if UV is irradiated from the upper face, the UV is blocked bythe metal layer B, and UV does not reach the adhesive layer C.

M represents a mask, which blocks regions other than regions in which UVirradiation is necessary, so that areas not requiring UV irradiation arenot irradiated. When a layer which does not pass UV exists on therear-face side, such method of irradiating from the rear-face sidecannot be adopted.

When performing UV irradiation so that only transfer target portions areirradiated with UV, in addition to the method of using the mask M toblock regions other than regions for irradiation as shown in the exampleabove, for example a method of concentrating light using a lens or thelike, or a method of using laser light with good directionality, orsimilar can be adopted.

Finally, the multilayer film F is peeled and removed, as shown in (c) ofFIG. 4. At this time, in portions in which the adhesive layer C ishardened, the metal layer B is transferred to the side of the inclinedface 5 via the adhesive layer C, and portions not hardened are peeledand removed together with the base A.

Next, FIG. 5 is a conceptual diagram showing an example of the methodwhen the adhesive layer C is hardened after transfer. As an example ofthermosetting, only a method of heating the entire board is shown, andas an example of UV hardening, a method of hardening without pressingthe head H from above the metal layer B is shown.

When the adhesive layer C comprises a material which has adhesiveproperties, or which exhibits adhesive properties upon heating, theadhesive layer C can be hardened after the metal layer B has beentransferred onto the inclined face 5. That is, while depending on thetypes and combinations of materials, in the state in which the adhesivelayer C has not yet hardened, if the adhesive force between the inclinedface and the adhesive layer exceeds the adhesive force between the metallayer and the base film, then the metal layer B can be transferred tothe inclined face 5 by peeling away the base A even when the adhesivelayer C is not hardened.

In the method shown in FIG. 5, similarly to the explanation using FIG. 4above, the multilayer film F comprising the base A, metal layer B, andadhesive layer C is set on the surface of the inclined face 5 for mirrorformation, to which transfer is to be performed ((a) of FIG. 5). At thistime, the adhesive layer C is held so as to face the inclined face 5.Then, the head H, the tip portion of which preferably comprises anelastic material, is brought close to the side of the base A of themultilayer film F, and the multilayer film F is pressed against theinclined face 5 ((b) of FIG. 5), and then the multilayer film F ispeeled and removed to cause transfer of the metal layer B onto theinclined face 5 ((c) of FIG. 5), and finally the adhesive layer C issubjected to thermosetting ((d-1) of FIG. 5) or to UV hardening ((d-2)of FIG. 5).

When, while causing thermosetting, heating is performed through themetal layer B from the heating head H, the temperature of the heatinghead H is preferably set to the temperature of 1 to 20% higher than thetemperature to which heating is required in order to compensate fordispersion of heat from the metal layer B, similarly to the case of FIG.4. However, when heating the entire board, this is not necessary. Thetiming for thermosetting may be immediately after the transfer, or maybe after formation of the second cladding layer.

When inducing UV hardening also, there are in essence no differencesfrom the example shown in FIG. 4 above, and UV hardening may beperformed similarly. The timing of UV hardening may also be eitherimmediately after transfer, or in an arbitrary stage after formation ofthe second cladding. However, when UV transmissivity from the rear faceis lost due to coverlay formation or for some other reasons, UVhardening is to be performed in advance. When inducing UV hardeningimmediately after transfer, it is useful to perform UV irradiation whileagain pressing the head against the transferred metal face B, in view ofsecuring stability of the transferred face.

As explained above, when executing methods such as those shown in FIG. 4and FIG. 5, it is not necessarily required that the adhesive layer C ishardened at the time the metal layer B is transferred onto the inclinedface 5, but it is possible to harden by UV treatment, heat treatment, orother means after transfer.

However, if the adhesive layer C is in the unhardened state, the metallayer B is pulled toward the side of the base A when the base A ispeeled, and as a result the flatness of the final mirror face isworsened, possibly resulting in light scattering losses. Hence in orderto avoid such a possibility and obtain stable reflectivity, it isdesirable that the base A be peeled away after the adhesive layer C hasbeen hardened and the metal layer B be transferred onto the inclinedface 5.

As already stated, although examples of the preferred adhesive layersinclude thermosetting layers and UV hardening layers, thermosettingmaterials are more preferred as those for the adhesive layer in thepresent embodiment. When causing hardening of the adhesive layer, it isideal that only the adhesive layer in contact with the inclined face ishardened. In order for example to achieve it with the UV hardeningadhesive layer, either the light is to be condensed in spots, or themask is to be used to limit the irradiated area. Also, if irradiationwith light is performed from the base side, the metal layer is in theway and the UV does not penetrate, so that irradiation must be performedfrom the rear side. Thus, the methods of hardening tend to becomecomplex. However, if the thermosetting materials are used for theadhesive layer, heating only in portions can be performed comparativelyeasily by using the heating head such as a soldering iron for example.

As the device for effecting transfer of the metal layer B of themultilayer film F onto the inclined face for mirror-reflecting filmformation, no limitations in particular are imposed, and any device maybe used which presses the adhesive layer side of the multilayer filmagainst the inclined face for mirror formation and, by applying pressureand/or heat from the base side of the multilayer film, causes transferand bonding of the metal layer to the inclined face. Among such devices,a device comprising the heating head H, a camera to recognize mirrorformation positions, and a stage capable of moving with submicronprecision, is preferred.

For example, as shown in (b) of FIG. 4, when the adhesive layer C is athermosetting type layer, the device is preferable which comprises theheating head H capable of pressing against the inclined face 5 from theside of the base A of the multilayer film F and applying heat andpressure. Or, as shown in (b-1) and (b-2) of FIG. 4, when the adhesivelayer C is a UV hardening layer, the device is preferable whichcomprises the head H capable of pressing closely against the inclinedface 5 from the side of the base A of the multilayer film F.

Examples of the simple device comprising the heating head H include atemperature-controlled type solder iron, the tip temperature of whichcan be adjusted so as to be from 1 to 20% higher than the thermosettingtemperature of the adhesive layer C. And, as explained below, when asolder iron is used having an elastic member mounted on the tip portionof the iron, temperature adjustment can be performed such that atemperature of the surface of the elastic member is at the prescribedtemperature by additionally setting a thermocouple in place.

In order to press the multilayer film F against the inclined face 5using the head H (or heating head H) while fixed in place and causehardening of the adhesive layer C, it is preferable that the head H (orheating head H) has a pressing force of from 0.05 MPa to 0.2 MPa.

Further, it is desirable that the head in the device used for pressing amultilayer film against an inclined face for mirror formation andcausing transfer of the metal layer has an elastic tip. The reason forthis is as follows.

That is, the material for the waveguide comprising the inclined face formirror formation is generally inelastic, and moreover the inclined faceis of course inclined relative to the board face. Hence, if the head tipis in a flat shape with no elasticity, it is necessary to make fineadjustments to the head orientation in order to apply pressure and heatto the inclined face uniformly. If the tip face of the head is notparallel to the inclined face upon making contact, pressure and heat areconveyed unevenly, and as a result the mirror performance is worsened.

While depending on the machining method, the inclined face itself is notnecessarily flat and may have a degree of undulation or similar, and theextent of undulations or similar is also often not constant. It istherefore not practical to prepare a head with a tip shape whichaccommodates the various undulations or similar.

Further, if the head with an elastic tip is used, as for example shownin FIG. 6 (in which H represents the head and Hd indicates the elasticmember), the tip can flexibly follow the undulations or similar of theinclined face 5, and pressure and heat can be applied uniformly and withstability, thus such head being preferable.

Examples of such elastic materials include, but are not limited to,silicone rubber, nitrile rubber, fluoride rubber, and the like. When thematerials are used for a heating head, an elastic material havingadequate thermal resistance according to the temperature may beselected.

As the metal layer, it is desirable that a metal layer having highreflectivity at 850 nm be used. This is because the mirror can beoptimized at 850 nm, which is the wavelength of the VCSELs(Vertical-Cavity Surface-Emitting Lasers) which are generally used aslight sources of optical wiring boards.

Further, in the multilayer film used in the embodiment, it is preferablethat the metal layer be divided into a prescribed shape, which is alsoone preferred embodiment when implementing the invention.

That is, when transferring the metal layer to the inclined face with theadhesive layer intervening, the metal layer is caused to be separatedfrom the base film. At this time, the unhardened adhesive layer in otherportions than those for transfer and the metal layer which is layeredthereupon are required to be simultaneously cut, peeled and removed.Although the metal layer can be cut comparatively easily since it isextremely thin, in order to enable still easier cutting, it ispreferable that the metal layer be divided into the prescribed shape inadvance.

FIG. 7 is a diagram showing an example of such a preferred multilayerfilm F for transfer. Here, the prescribed shape is a shape whichadequately covers the entire inclined face for mirror formation to whichtransfer is to be performed. For example, a shape which is substantiallythe same as or slightly larger than the shape of the inclined face, ispreferred. In the example of FIG. 7, metal layers B with substantiallysquare shapes are provided on the base A, at equal intervals verticallyand horizontally, and the adhesive layer C for transfer is formed on thesurface side thereof.

In FIG. 8, (a) through (d) are diagrams showing in an unlimited manneran example of a method of fabricating the preferred multilayer film asshown in FIG. 7. The base A such as a PET film is prepared ((a) of FIG.8), and the metal layer B with the desired thickness is formed on thesurface thereof in the desired shape and position by evaporationdeposition, sputtering, or similar ((b) of FIG. 8). At this time, amethod is employed in which the shape and positions of the metal layer Bare regulated by using the mask M, so that the metal layer B is notformed in areas which are to be divided.

Then, similarly to the method shown in FIG. 3 above, a varnish forformation of the adhesive layer C is applied onto the metal layer B ((c)of FIG. 8), and then the varnish is dried to obtain a multilayer film Ffor reflecting film formation ((d) of FIG. 8).

The example shown in FIG. 8 is not such as to limit the invention, andthe shape, positioning, and the like of the metal layer B can bemodified arbitrarily according to the structure and othercharacteristics of the surface layer circuit of the optical wiringboard.

As the constituent material of the adhesive layer C, a material withhigh transmissivity for light at 850 nm is preferably used, which is onepreferred embodiment for implementing the invention. The reason for thisis that, as explained above, the wavelength of VCSELs which aregenerally used as light sources in optical wiring boards is 850 nm, andthe mirror can be optimized in this wavelength region.

When the metal layer B formed by transfer reflects light as a mirror,the light passes through the adhesive layer C as shown in FIG. 9 (thediagram showing the manner in which light passes through the adhesivelayer during mirror reflection). Thus, if the optical transmissivity ofthe adhesive layer C is heightened, reflected light losses can besuppressed. Here, a “high” transmissivity for light is defined so that aloss in transmission is preferably 0.5 dB or less.

For the similar sense, it is desirable that a material withsubstantially the same refractive index as the core layer of thewaveguide is used as the constituent material of the adhesive layer C.This is because, when the metal layer B reflects light as a mirror, thelight passes through the adhesive layer C as shown in FIG. 9, and soreflection losses at the contact interface can be suppressed. Here, therefractive index indicates the refractive index at the wavelength of thepropagating light, and “substantially the same” means that a differencein the refractive indexes is within approximately 1%.

As described above, the explanation has focused on the main constitutionof the invention which is the method of forming the metal layer on theinclined face for mirror formation. However, the above explanation isprovided only to illustrate the present invention by embodiments in allaspects, and thus the present invention is not limited thereto. It is tobe understood that numerous modifications not exemplified here are alsopossible without departing from the scope of the present invention.

The optical wiring board comprising the mirror-reflecting film obtainedby adopting the methods of the present invention can be manufactured ata lower cost compared with conventional optical wiring boards due to theadvantages of the manufacturing method, and can moreover afford superiorperformance as well. Hence the optical wiring board having themirror-reflecting film manufactured by this method is also includedwithin the technical scope of the invention.

EXAMPLES

Below, the invention is explained in greater detail referring toexamples. However, the invention is not limited by the followingexamples, and appropriate modifications can be made within the scope ofthe embodiments as described above and below. These modifications arealso included in the technical scope of the invention.

(Preparation of Epoxy Film for Optical Waveguide)

First, the following three types of film were prepared as materials forthe optical waveguide.

1) Epoxy Film A

A blend of 7 parts by weight of a polypropylene glycol glycidyl ether(product name “PG207” manufactured by Tohto Kasei Co., Ltd.), 25 partsby weight of a liquid-form hydrogenated bis-phenol A type epoxy resin(product name “YX8000” manufactured by Japan Epoxy Resin Co., Ltd.), 20parts by weight of a solid hydrogenated bis-phenol A type epoxy resin(product name “YL7170” manufactured by Japan Epoxy Resin Co., Ltd.), 8parts by weight of 1,2-epoxy-4-(2-oxiranyl)cyclohexane additive of2-2-bis(hydroxy methyl)-1-butanol (product name “EHPE3150”, manufacturedby Daicel Chemical Industries, Ltd.), 2 parts by weight of a solidbis-phenol A epoxy resin (product name “Epicoat 1006FS”, manufactured byJapan Epoxy Resin Co., Ltd.), 20 parts by weight of a phenoxy resin(product name “YP50”, manufactured by Tohto Kasei Co., Ltd.), 0.5 partsby weight of a cationic photo-hardening initiator (product name “SP170”,manufactured by Adeka Corp.), 0.5 parts by weight of a cationicthermo-hardening initiator (product name “SI-150L”, manufactured bySanshin Chemical Industry Co., Ltd.), and 0.1 parts by weight of asurface conditioner (product name “F470”, manufactured by DIC Corp.),was dissolved in a solvent of 30 parts by weight toluene and 70 parts byweight methyl ethyl ketone (MEK), and after filtering using a membranefilter with a hole diameter of 1 μm, vacuum degassing was performed toobtain an epoxy resin varnish A.

The epoxy resin varnish A thus obtained was applied using a bar coateronto PET film of thickness 50 μm, and after primary drying for 10minutes at 80° C., secondary drying for 10 minutes at 120° C. wasperformed. Finally, as a protective film, an OPP covering film 35 μmthick was applied, to obtain an epoxy film A of film thickness 15 μm.The refractive index of the epoxy film A at 579 nm was 1.54.

2) Epoxy Film B

A blend of 42 parts by weight of a liquid form bis-phenol A epoxy resin(product name “Epichron 850S”, manufactured by DIC Corp.), 55 parts byweight of a solid bis-phenol A epoxy resin (product name “Epicoat1006FS”, manufactured by Japan Epoxy Resin Co., Ltd.), 3 parts by weightof a phenoxy resin (product name “YP50”, manufactured by Tohto KaseiCo., Ltd.), 1 part by weight of a cationic photo-hardening initiator(product name “SP170”, manufactured by Adeka Corp.), and 0.1 part byweight of a surface conditioner (product name “F470”, manufactured byDIC Corp.), was dissolved in a solvent of 24 parts by weight toluene and56 parts by weight MEK, and after filtering using a membrane filter witha hole diameter of 1 μm, vacuum degassing was performed to obtain anepoxy resin varnish B.

The epoxy resin varnish B was formed into a film similarly to the aboveprocess, and an epoxy film B of film thickness 40 μl was prepared. Therefractive index of the epoxy film B at 579 nm was 1.59. Upon evaluatingthe optical transmissivity at 850 nm, adequate transparency of 0.06dB/cm was determined.

3) Epoxy Film C

The epoxy resin varnish A used in preparing the epoxy film A above wasemployed, and by forming into a film similarly to the process describedabove, an epoxy film C of film thickness 55 μm was prepared. Therefractive index of the epoxy film C at 579 nm was 1.54.

(Fabrication of Multilayer Film for Thermal Transfer)

Next, the multilayer film for thermal transfer, for use in formation ofthe mirror-reflecting film, was fabricated by the following procedure.

(Multilayer Film 1 for Transfer)

In the method of preparation of the epoxy resin varnish B used inpreparing the epoxy film B, in place of the 1 part by weight of thecationic photo-hardening initiator “SP170” (same as above), 1 part byweight of the cationic thermo-hardening initiator “SI-150L” (same asabove) was blended; otherwise the same method was used to fabricate thevarnish for adhesive layer formation.

The varnish thus obtained was used to form a film similarly to thepreparation of the epoxy film B, and upon evaluating the refractiveindex and transmissivity at 579 nm for the film, values of 1.59 and 0.06dB/cm respectively were obtained, which were the same as for the epoxyfilm B.

A polyimide film of thickness 25 μm (product name “Kapton 100H”,manufactured by Du Pont-Toray Co., Ltd.) was cut into a square shapemeasuring 100 mm on a side. On one face, a thin film of Cu was formed bya vacuum sputtering method. The thickness of the thin film thus formedwas 1500 Å.

On the Cu side of the two-layer structure film thus obtained, thevarnish for adhesive layer formation obtained above was applied using abar coater, and then primary drying for 10 minutes at 80° C. wasperformed, followed by secondary drying for 10 minutes at 120° C., toform an epoxy layer (adhesive layer) of thickness 10 μm.

(Multilayer Film 2 for Transfer)

Similarly to the multilayer film 1 for transfer described above, apolyimide film of thickness 25 μm was cut into a square shape measuring100 mm on a side, and a vacuum sputtering method was used to form a Cuthin film on one face. When forming the Cu thin film by the vacuumsputtering method, a mask with lattice-shape openings was fixed abovethe polyimide film. The opening portions of the mask were in squareshapes 45 μm on a side, and the pitch from opening to opening was 65 μm.The two-layer film obtained had a Cu layer formed into a lattice ofsquare shapes 45 μm on a side. On the Cu side of the two-layer film wasformed an adhesive layer, similarly to the multilayer film 1 fortransfer.

(Multilayer Film 3 for Transfer)

A multilayer film 3 for transfer was obtained in the same way as themethod of fabrication of the multilayer film 1 for transfer describedabove, except that Al was used in place of Cu as the metal used invacuum sputtering. The thickness of the film was likewise 1500 Å.

Example 1

As a copper-plated flexible board, a FELIOS board, manufactured byPanasonic Electric Works Co., Ltd. (product name “R-F775”, polyimidethickness 25 μm, two-sided board) was made available, and all the Cu onone face was etched off.

As the material for cladding, the epoxy film A was used; the OPP film ofthe film A was peeled away, the film was placed such that the materialwas in contact with the etched-off face of the board, and pressure wasapplied at 60° C. and in 0.2 MPa for 120 seconds to perform lamination.Next, a super-high pressure mercury lamp was used for irradiation with2000 mJ of light at 365 nm to cause hardening, and finally the PET filmwas peeled away to form the lower cladding on the board.

Next, the epoxy film B was used as the core material, and similarly tothe above procedure, the epoxy film B was placed on the surface of thelower cladding, and lamination was performed by applying pressure at 60°C. and in 0.2 MPa for 120 seconds. Next, a photomask provided with 20slits of width 40 μm and length 110 mm at intervals of 250 μm was used;this was brought into close contact with the surface of the epoxy filmB, and by irradiating to 2000 mJ with ultraviolet rays at 365 nm from asuper-high pressure mercury lamp, with the light adjusted to obtain aparallel beam, portions of the transparent epoxy film B corresponding tothe slits were hardened with the ultraviolet rays.

Thereafter, a Freon-substitute water-based cleaning agent (product name“Cleanthrough”, manufactured by Kao Corp.) was used as a developingliquid in development, to form the core.

Next, micromirror formation was performed. First, dicing was used toform an inclined face at 45°. The blade used was a metal bonded bladewith a tip angle of 90°, the abrasive used was #5000, and the blade waslowered from directly above the mirror formation portion at a speed of0.03 mm/s while maintaining a rotation rate of 15,000 rpm, to cut thecore to a position of approximately 5 μm cutting into the lowercladding. The blade was then made to travel in the horizontal directionat a traveling speed of 5 mm/s, and after cutting, the rotating bladewas raised in the vertical direction at a speed of 0.03 mm/s. At thistime, the face roughness of the cut face was on average 100 nm (rms).Next, thermal transfer was performed to form the reflecting film on theinclined face. As the multilayer film for transfer, multilayer film 1was used. And, as the head for thermal transfer, atemperature-controlled solder iron was used.

The multilayer film 1 was held with tweezers and, while observing with amicroscope, was positioned such that the adhesive side of the multilayerfilm 1 was in contact with the inclined face at 45° obtained by dicing.Then, the soldering iron with tip temperature adjusted to 170° C. wasbrought into contact for five seconds from the base side of themultilayer film 1 to cause hardening of the adhesive layer. Then, whenthe multilayer film 1 was peeled and removed, only the polyimide filmwas peeled from the 45° inclined face, and it was confirmed that the Cu,together with the adhesive layer, was bonded to the inclined face.

Upon measuring the surface roughness of the transferred face at thistime, an RMS value of 80 nm was obtained.

Next, the upper cladding was formed. Similarly to the formation of thelower cladding described above, the epoxy film C was placed onto thecore from above, and after laminating by applying pressure of 0.2 MPafor 120 seconds at 80° C., irradiation to 2000 mJ (at 365 nm) with lightfrom a super-high pressure mercury lamp was performed to causehardening, and finally by peeling the PET film, the upper cladding wasformed.

Finally, electrical circuit formation was performed by etching the Culayer on the surface, to complete the flexible-type optical wiringboard.

For the optical wiring board obtained in this way, by causing light ofwavelength 850 nm to enter from a mirror on one side and receiving thelight emitted from an another mirror by using a photodiode (PD), lossesfrom one mirror to the other mirror were evaluated, and as the result,the average value for 20 lights was 5.2 [dB].

Example 2

Methods similar to those of Example 1 were used, up to formation of the45° inclined faces.

Next, reflecting film formation by thermal transfer was performed. Asthe multilayer film for transfer, the multilayer film 3 was used, and asthe head for thermal transfer, a temperature-controlled type solderingiron similar to that of Example 1 was used.

The multilayer film 3 was held with tweezers, and while using amicroscope for observation, was positioned such that the side of theadhesive layer of the multilayer film 3 was in contact with the 45°inclined face obtained by dicing. The soldering iron with the tiptemperature adjusted so as to be 170° C. was brought into contact for 5seconds with the base face side of the multilayer film 3 to causehardening of the adhesive layer. Then, upon peeling and removing themultilayer film 3, only the polyimide film was peeled from the 45°inclined face, and it was confirmed that Al was transferred and bondedto the inclined face.

Upon measuring the surface roughness of the transferred face at thistime, an RMS value of 82 nm was obtained.

Then, the electrical circuit was formed similarly to Example 1, tocomplete the flexible-type optical wiring board.

Light of wavelength 850 nm was entered from a mirror on one side of theoptical wiring board obtained in this way, and upon using a PD toreceive the light emitted from another mirror, and evaluating lossesfrom one mirror to the other mirror, the average value for 20 lights was7.8 [dB]. Judging from this value, optical losses are worsened comparedwith Example 1; this is attributed to the fact that the reflectivity at850 nm of Al, which was used as the metal layer, is poorer than that forCu (absorption by the metal is larger).

Example 3

Similarly to Example 1, fabrication up to formation of the 45° inclinedfaces was performed.

Next, reflecting film formation by thermal transfer was performed. Asthe multilayer film for transfer, the multilayer film 1 was used. As thesoldering iron for thermal transfer, the same temperature-controlledtype soldering iron as in Example 1 was used; silicone rubber ofthickness 15 μm was mounted so as to cover the tip of the iron, and athermocouple was installed.

The multilayer film 1 was held with tweezers, and while using amicroscope for observation, was positioned such that the side of theadhesive layer of the multilayer film 1 was in contact with the 45°inclined face obtained by dicing. The soldering iron, in which a tiptemperature was adjusted such that the thermocouple readout was 170° C.(the silicone rubber surface was at 170° C.), was brought into contactfor 5 seconds with the base face side to cause hardening of the adhesivelayer. Then, upon peeling and removing the multilayer film 1, only thepolyimide film was peeled from the 45° inclined face, and it wasconfirmed that Cu was transferred and bonded to the inclined face.

Upon measuring the surface roughness of the transferred face at thistime, an RMS value of 70 nm was obtained.

Then, the electrical circuit was formed similarly to Example 1, tocomplete the flexible-type optical wiring board.

Light of wavelength 850 nm was entered from a mirror on one side of theoptical wiring board obtained in this way, and upon using a PD toreceive the light emitted from another mirror, and evaluating lossesfrom one mirror to the other mirror, the average value for 20 lights was4.8 [dB]. Judging from this value, optical losses are improved comparedwith Example 1; this is attributed to the fact that the elastic memberwas affixed to the tip of the iron, so that the surface roughness of theCu layer at the time of thermal transfer was improved.

Example 4

Similarly to Example 1, fabrication up to formation of the 45° inclinedfaces was performed.

Next, reflecting film formation using the multilayer film for thermaltransfer was performed. As the multilayer film, the multilayer film 2described above was used. As the soldering iron for thermal transfer,the same temperature-controlled type soldering iron as in Example 1 wasused; silicone rubber of thickness 15 μm was mounted so as to cover thetip of the iron, and a thermocouple was installed.

The multilayer film 2 was held with tweezers, and while using amicroscope for observation, was positioned such that the side of theadhesive layer of the multilayer film 2 was in contact with the 45°inclined face obtained by dicing. The soldering iron, in which a tiptemperature adjusted such that the thermocouple readout was 170° C. (thesilicone rubber surface was at 170° C.), was brought into contact for 5seconds with the base face side of the multilayer film 2 to causehardening of the adhesive layer. Then, upon peeling and removing themultilayer film, only the polyimide film which was the base layer waspeeled from the 45° inclined face, and it was confirmed that Cu wasbonded to the inclined face. In this Example, due to the advantageouseffect of the divided portions provided in the metal layer, film peelingand removal were performed more easily than in Example 3.

Upon measuring the surface roughness of the transferred face at thistime, an RMS value of 63 nm was obtained.

Then, the electrical circuit was formed similarly to Example 1, tocomplete the flexible-type optical wiring board.

Light of wavelength 850 nm was entered from a mirror on one side of theoptical wiring board obtained in this way, and upon using a PD toreceive the light emitted from another mirror, and evaluating lossesfrom one mirror to the other mirror, the average value for 20 lights was4.5 [dB]. Judging from this value, optical losses are improved comparedwith Example 1; this is attributed to the fact that the metal layer ofthe multilayer film was divided in advance, so that surface roughness atthe time of transfer was improved.

Comparative Example 1

Similarly to Example 1, fabrication up to formation of the 45° inclinedfaces was performed.

Next, a vacuum sputtering method was used to form a Cu reflecting film.A mask with openings only at positions corresponding to 45° inclinedfaces was used, and the Cu film formation was adjusted so that Cuadhered to only the desired places. The Cu thickness was 1500 Å.

Then, the electrical circuit was formed similarly to Example 1, tocomplete the flexible-type optical wiring board.

Light of wavelength 850 nm was entered from a mirror on one side of theoptical wiring board obtained in this way, and upon using a PD toreceive the light emitted from another mirror, and evaluating lossesfrom one mirror to the other mirror, the average value for 20 lights was5.9 [dB].

Comparative Example 2

Similarly to Example 1, fabrication up to formation of the 45° inclinedfaces was performed.

In order to improve the smoothness of the 45° inclined faces obtained, aTEA-CO₂ laser (wavelength 9.8 μm) was used in irradiation to theinclined faces from the normal direction, at an energy density of 9mJ/mm², over irradiated areas 100 μm on a side, in four irradiationpulses with pulse widths of 9.3 μs repeated at a frequency of 100 Hz.The average (rms) surface roughness of the cut faces at this time was 60nm, which was an improvement compared with the result of dicing alone.

Then, a Cu reflecting film was formed by a vacuum sputtering methodsimilarly to Comparative Example 1, and thereafter, the electricalcircuit was formed similarly to Example 1, to complete the flexible-typeoptical wiring board.

Light of wavelength 850 nm was entered from a mirror on one side of theoptical wiring board obtained in this way, and upon using a PD toreceive the light emitted from another mirror, and evaluating lossesfrom one mirror to the other mirror, the average value for 20 lights was4.4 [dB], which shows that optical losses were lower than forComparative Example 1.

As described above, one aspect of the present invention is directed to amethod of forming a mirror-reflecting film on a waveguide in an opticalwiring board, the method being characterized in that a multilayer film,in which a base, a metal layer and an adhesive layer are layered in thisorder, is used, and in that the metal layer is transferred and bonded toan inclined face for mirror-reflecting film formation provided on thewaveguide, with the adhesive layer of the multilayer film intervening.

According to this method, when forming the mirror-reflecting film on thewaveguide in the optical wiring board, the mirror-reflecting film can beformed easily and inexpensively using the smallest quantity of metalpossible and employing comparatively simple facilities and techniques.

When executing the above method, by hardening the adhesive layer, themetal layer can be transferred and bonded to the inclined face formirror-reflecting film formation. In the method, a thermosettingadhesive layer is preferably used as the adhesive layer.

The preferred method of transferring and bonding the metal layer of themultilayer film to the inclined face comprises pressing the adhesivelayer side of the multilayer film against the inclined face formirror-reflecting film formation, and applying pressure and/or heat fromthe base side of the multilayer film using a head with an elastic tip.

In the method, as the metal layer, at least one kind of metal selectedfrom the group consisting of copper, silver, and gold is preferablyused.

It is another preferred embodiment in the method to use the multilayerfilm in which the metal layer is divided into a prescribed shapeaccording to a shape of the inclined face for mirror-reflecting filmformation.

As the adhesive layer in the method, it is desirable to use a materialwhich transmits light of wavelength 850 nm.

It is also a preferred embodiment to use, as the adhesive layer,material which has substantially the same refractive index as a corelayer of the waveguide.

Further, an optical wiring board comprising the mirror-reflecting filmfabricated by the method described above is also included within thetechnical scope of the present invention.

INDUSTRIAL APPLICABILITY

According to the present invention, when forming a mirror-reflectingfilm on a waveguide in an optical wiring board, the mirror-reflectingfilm can be formed easily and inexpensively using the smallest quantityof metal possible and employing comparatively simple facilities andtechniques.

The invention claimed is:
 1. A method for forming a mirror-reflecting film on a waveguide in an optical wiring board, the method comprising: providing a waveguide with an inclined face providing a multilayer film, the multilayer film comprising a base layer, a metal layer and an adhesive layer that are layered in this order, and transferring and bonding, after the providing of the waveguide and of the multilayer film, the metal layer to the inclined face for mirror-reflecting film formation provided on the waveguide, with the adhesive layer of the multilayer film intervening between the metal layer and the inclined face, the base layer having a uniform thickness.
 2. The method for forming a mirror-reflecting film according to claim 1, wherein the transferring and bonding of the metal layer to the inclined face for mirror-reflecting film formation comprises hardening of the adhesive layer.
 3. The method for forming a mirror-reflecting film according to claim 1, wherein a thermosetting adhesive layer is used as the adhesive layer.
 4. The method for forming a mirror-reflecting film according to claim 1, wherein the transferring and bonding of the metal layer to the inclined face comprises pressing the adhesive layer side of the multilayer film against the inclined face for mirror-reflecting film formation, and applying pressure and/or heat from the base layer side of the multilayer film using a head, wherein a tip of the head comprises an elastic member.
 5. The method for forming a mirror-reflecting film according to claim 1, wherein the metal layer comprises at least one of copper, silver, and gold.
 6. The method for forming a mirror-reflecting film according to claim 1, further comprising dividing the metal layer of the multilayer film into a prescribed shape according to a shape of the inclined face for mirror-reflecting film formation.
 7. The method for forming a mirror-reflecting film according to claim 1, wherein a material which transmits light of wavelength 850 nm is used as the adhesive layer.
 8. The method for forming a mirror-reflecting film according to claim 1, wherein a material with substantially the same refractive index as a core layer of the waveguide is used as the adhesive layer.
 9. An optical wiring board, comprising the mirror-reflecting film fabricated by the method according to claim
 1. 10. The method for forming a mirror reflecting film on a waveguide according to claim 1, wherein an entire surface of the metal layer is covered by the adhesive layer.
 11. The method for forming a mirror reflecting film on a waveguide according to claim 1, wherein the metal layer covers an entire surface of the inclined face.
 12. The method for forming a mirror reflecting film on a waveguide according to claim 1, wherein the inclined face is uniformly covered by the metal and adhesive layers.
 13. The method for forming a mirror reflecting film on a waveguide according to claim 1, further comprising peeling the base layer away from the metal layer bonded to the inclined face.
 14. The method for forming a mirror-reflecting film on a waveguide according to claim 1, wherein the provided multilayer film, comprising the base layer, the metal layer and the adhesive layer are positioned on the inclined face. 